The present invention relates to processes and installations for the heat treatment of metal wires, and more particularly carbon steel wires, these wires being used to reinforce articles of rubber and/or of plastic material or materials, for instance pneumatic tires.
These heat treatments have the purpose, on the one hand, of increasing the wiredrawing capability of the wires and, on the other hand, of improving their mechanical properties and their endurance.
The known treatments of this type comprise two phases:
a first phase which consists in heating the wire and maintaining the wire at a temperature above the AC.sub.3 transformation temperature to obtain a homogeneous austenite: PA1 a second phase which consists in cooling the wire to obtain a fine pearlite structure. PA1 (a) the wire, which has been previously maintained at a temperature above the AC.sub.3 transformation temperature to obtain a homogeneous austenite, is cooled until the wire reaches a given temperature which is below the AC.sub.3 transformation temperature and above the temperature of the nose of the curve of the start of the transformation of metastable austenite into pearlite, the wire then having a metastable austenite structure without pearlite; PA1 (b) then regulating the temperature of the wire to not more than 10.degree. C. above or below said given temperature, this regulation being obtained by passing an electric current through the wire for a period of time greater than the pearlitization time and by effecting a modulated ventilation for a part of this time; PA1 (c) then cooling the wire. PA1 (a) means for cooling the wire which has been previously maintained at a temperature above the AC.sub.3 transformation temperature, these cooling means permitting the wire to reach a given temperature which is below the AC.sub.1 transformation temperature and above the temperature of the nose of the curve of the start of the transformation of metastable austenite into pearlite, the wire then having a metastable austenite structure without pearlite; PA1 (b) means for then regulating the temperature of the wire to not more than 10.degree. C. above or below said given temperature for a period of time greater than the pearlitization time, these regulating means comprising electric means for passing an electric current through the wire and means for modulated ventilation of the wire; PA1 (c) means for then cooling the wire.
One of the most common of these processes is a heat treatment known as "patenting" which consists of an austenitizing of the wire at a temperature of 800.degree. to 950.degree. C. followed by immersion in a bath of molten lead or salts maintained at a temperature of 450.degree. to 600.degree. C.
The good results obtained, particularly in the case of the heat treatment with lead, are generally attributed to the fact that the very high coefficients of convection which are obtained between the wire and the cooling fluid permit, on the one hand, a rapid cooling of the wire between the AC.sub.3 transformation temperature and a temperature slightly higher than that of the lead and, on the other hand, a limiting of the "recalescence" during the transformation of the metastable austenite into pearlite, the recalescence being an increase in the temperature of the wire due to the fact that the energy contributed by the metallurgical transformation is greater than the energy lost by radiation and convection.
Patenting, unfortunately, results in high costs since the handling of liquid metals or molten salts leads to cumbersome technologies and the necessity of cleaning the wire after the patenting.
Furthermore, lead is very toxic and the health problems to which it gives rise lead to substantial expenses.